This invention relates to video systems, and more specifically, to gamma correction of video intensity values for causing a linear range of video intensity values to be displayed on a cathode ray tube screen as a linear gradation of color intensities.
In a video display, the relation of the magnitude of the output signal, y, to the magnitude of the input signal, x, is expressed in the relationship y=f(x), where f(x) is some non-linear function due to the non-linear response of the cathode ray tube (CRT), as well as any other non-linearities that occur in the system. This relation is most easily expressed in normalized form as f(x)=xxcex3, with the parameter xcex3 incorporating the composite non-linearities of the signal reproduction. In an ideal, distortionless system, the overall value of gamma would be unity, xcex3=1, and the relationship between the brightness of a portion of an object and the brightness of the corresponding image would be strictly proportional.
To compensate for this non-linear response, the techniques of gamma correction are used. CRT gamma correction is a complex task that is highly dependent a manufacturer""s color philosophy. Traditionally, analog methods are used to perform gamma correction by providing phosphor characteristics equations, thereby letting the manufacture set the xcex3 values. The needed correction can then be explicitly computed. However, using this method for digital video signals is complicated, as the gamma correction function is exponential, and digital correction can be time consuming. Furthermore, this method would limit the manufacture to the supplied correction curve and take away any flexibility in the correction process.
Another method of performing gamma correction on a digitized intensity signal is to translate each of the N-bit red, green, and blue color intensity values to compensated N-bit color intensity values by using a color lookup table. This allows the manufacturer to program the lookup table to any desired values, using their digital television controller in TV applications or by storing a set in of values in ROM or other memory for computer graphic applications. The lookup table is typically stored in a solid state memory and includes a range of color intensity values, each of which is associated with a corresponding gamma corrected value. These gamma corrected values are derived from the functional relationship above and stored in the lookup table. The gamma corrected values read from the table are then converted into analog intensity signals, which are then applied to a control grid of the CRT.
In the lookup table approach, the integrated circuit must be designed with enough additional memory to support a full lookup table as well as any digital television controller interconnect and ROM requirements. For N-bit color, this requires 2Nxc3x97N bits of memory for each color. For the example of 10-bit RGB color, this amounts to 3xc3x971024xc3x9710 bits, a very large and costly amount of memory space to place on a television controller.
A method of CRT gamma correction of n-bit color is presented which employs a reduced size lookup table. An n-bit input signal is separated into its m most significant bits and (nxe2x88x92m) least significant bits. Instead of using its full n-bit value as the input address into a 2nxc3x97n lookup table, a 2mxc3x97n table employing only the m most significant bits is used, thereby reducing the memory requirements for the lookup table by a factor of 2(nxe2x88x92m). Every cycle, two consecutive memory locations are read, starting from where in the lookup table the m most significant bits of the input signal are pointing. The output of the lookup table provides the n-bit gamma corrected values programmed in by the digital television controller or loaded from computer memory for each of these m-bit inputs. An approximation, preferably linear, of the gamma correction curve is then formed between these two output values and the n-bit, gamma corrected value of the full n-bit value of the input signal is then interpolated using its (nxe2x88x92m) least significant bits.
Additional objects, advantages, and features of the present invention will become apparent from the following description of its preferred embodiments, which description should be taken in conjunction with the accompanying drawings.